The main reason for undertaking this is my interest in space exploration and science, and the fact that Orbiter is closed source (and using D3D, which would be a problem even if it was open source).

I'm hoping to release this with a GPL license, though with a linking exception, allowing you to release closed source plug-ins.

I'd wish to support X11 (through SDL, GTK or GNUstep) and Mac OS X through a native interface written in Cocoa (of these, I will implement the Cocoa interface first and formost). I also intend on implement tests for relevant functionality with the OCUnit framework.

At the moment I am doing a pre-study of what have to be done. The basic outline of the project is as follows:

1. Celestial sphere simulator, i.e. the part that paints the sky. I intend on writing a FITS-parser (standard astronomical data format) to load star charts for this. Other than that, this shouldn't cause any overly big problems.

3. Flight models, and plug-in system allowing one to design new instruments and space craft, in custom dynamically loaded code.

I'd expect this project to go on for some considerable time, non the less it is certainly something that I'd like to do.

Anyway, I am interested in input about what one would expect from such a simulator. Also, any other relevant ideas would be greatly appreciated.

Ideas such as what kinds of scenarios do people expect to be included, e.g. moon landing, Apollo 13, emergency EVA-repair of micro meteorite induced leaks on spaceship / ISS (with a strict deadline naturally).

I am also looking for a GPL-compatible physics engine (although I could write my own if needed), is the ODE engine any good?

Space is big. You just won't believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it's a long way down the road to the drug store, but that's just peanuts to space. --Douglas Adams
For example travel time to the moon is ca. 3 Days. Towards Mars, our most interesting neighbour, you already need ca. 260 Days.
I never liked Games that are too real with space travel time for exactly this reason, sure you can speed it up by having a "skip" button, but why make the travel time so long and meaningless in the first time? What about reaching out to Pluto, shouldn't that feel like an accomplishment to the player? But if you can skip to the Mars, people will want to skip to Pluto too!
Maybe you thought of travel faster then light? When i make game ideas I think of stuff like: What will be the objectives, what the rewards, what the goals (both player and game goals). Can you explain these to me, I have never "played" orbiter, I imagine this simulation to be rather boring... Still maybe you're more of a simulation guy, and don't really want to make this a playable game?

Well, yes, going to the moon will require 3-4 days of simulated time which you can fast forward of course, though I intend to make a smart fast forward method that automatically slows down if anything interesting happens.

The objectives with Orbiter depends on the scenario. For example one scenario can be to take of from Cape Canaveral, reach orbit and dock with the space station. My idea is to have more exciting scenarios, but not without loosing realism.

Of course, the sim is not intended for the casual gamer, I mean, learning interplanetary navigation and how to enter a transfer orbit is rather difficult. However, if everything goes as planned, I can easily imagine walking / driving on the moon, and who knows what scenarios people will create.

I like the idea of entering a cruise or drift mode where for instance the camera zooms out of the ship and shows a nice cinematic flyby cutscene. you could then roll for random encounters (say...an explosion when stirring the oxygen tanks) and zoom back in with klaxons blaring.